Brinzolamide is a carbonic anhydrase inhibitor used to lower intraocular pressure in patients with ocular hypertension or open-angle glaucoma. Brinzolamide is chemically (R)-(+)-4-ethylamino-2-(3-methoxypropyl)-3,4-dihydro-2H-thieno[3,2-e]-1,2-thiazine-6-sulfonamide-1,1-dioxide and has the empirical formula C12H21N3O5S3. Brinzolamide has a molecular weight of 383.5 and a melting point of about 131° C.
This compound is disclosed in U.S. Pat. No. 5,378,703 (Dean, et al.). The compound is also disclosed in European patent EP 527801. U.S. Pat. No. 6,071,904 discloses processes for preparation of brinzolamide ophthalmic composition.
Brinzolamide in the form of ophthalmic suspension is developed and marketed by Alcon Laboratories Inc. in United States under the brand name Azopt® (brinzolamide ophthalmic suspension 1%). Brinzolamide is indicated for lowering elevated intra-ocular pressure (TOP) in patients with open-angle glaucoma or ocular hypertension (OHT).
Various methods have been disclosed in the prior art for the preparation of brinzolamide ophthalmic suspension. International patent application WO 98/25620 teaches that conventional sterilization methods cannot be employed in the manufacture of suspensions comprising brinzolamide since the compound recrystallizes as large needle-shaped crystals, upon cooling, after autoclaving.
According to WO 98/25620, dry heat sterilization is also not suitable, since it causes melting of the material, whereas sterilization by ethylene oxide and gamma irradiation introduces unacceptable degradation products.
EP0941094 discloses a process for making brinzolamide suspension by autoclaving of concentrated slurry of brinzolamide and tyloxapol; or brinzolamide and Triton X in milling bottle, and ball milling of the hot slurry after autoclaving, and then adding the slurry to the rest of the ingredients. It should be noted here that high temperatures and pressures of autoclave will dissolve brinzolamide. Later, when autoclaving is complete, upon cooling brinzolamide precipitates as large shaped crystals, having particle size of 1000 to 5000 μm. However, inclusion of tyloxapol and/or Triton X in the slurry allows the crystals to break up easily by ball milling. Brinzolamide cannot be administered as these large needle shaped crystals, as they will damage the eyes. Hence, precipitated brinzolamide crystals need to be milled to reduce their particle size.
Thus, the reference discloses autoclaving of the slurry of brinzolamide and surfactant and further ball milling the slurry. However, the drawback associated with this method is that it requires a milling bottle in which the slurry of brinzolamide could initially be autoclaved and then ball milled for further size reduction of needle shaped crystals of brinzolamide that are formed during autoclaving.
Dry heat sterilization causes melting of the material. Sterilization by ethylene oxide introduces unacceptable degradation products and residues, and sterilization by gamma irradiation of micronized material produces degradation products unacceptable for regulatory filing.
In most cases crystallization of active ingredients useful for ophthalmic use like carbonic anhydrase inhibitor, or others actives, occurs during preparation. Sterilization by autoclaving at temperature of 121° C. and 115 lbs of pressure leads to increase in solubility of the actives in the preparation and at that temperature brinzolamide goes into solution. However, upon cooling, brinzolamide precipitates as needle shaped crystals. These needle-shaped crystals are difficult to break and suspend. In different references either tyloxapol is used in solution so that the crystals are easier to break or special equipment such as ball mill and/or jet mill is used to break the large needle-shaped crystals.
The majority of the suspensions disclosed in the references faced the problem of crystallization and agglomeration of active ingredients during preparation as well as during storage. Crystallization or agglomeration of active leads to non-uniformity of dose, difficulty of administration, irritation to eye due to large drug particles and/or any ocular adverse effect due to high drug concentration.
So, there remains a need to formulate a dosage form in which drugs like brinzolamide, having low solubility can be solubilized or the drug is present in an amorphous form, to increase the permeability and bioavailability of the drug. None of the prior art disclosed above teaches about increased solubility of brinzolamide or converting it in an amorphous form.
The inventors of the present invention has formulated a sterile, ophthalmic pharmaceutical formulation, wherein when the active ingredient with low aqueous solubility (such as brinzolamide) in combination with polymers like Soluplus® and a surfactant like polysorbate 80, is either autoclaved or dissolved after heating above 50° C. Upon cooling, the active ingredient brinzolamide does not precipitate and stays in solution or in a partially amorphous form.